1. Field of the Invention
The present invention relates generally to an optical linear polymer material, and more particularly, to an optical polyimide precursor, an optical polyimide compound, and the fabricating methods thereof.
2. Description of the Related Art
Generally, optical linear high polymer materials are useful in a variety of optical devices that are the very core of the next-generation fast and large scale data communication industries. Such an optical linear high polymer material is typically utilized in fabricating optical devices for use purpose of optical wave direction, including opto-electronic integrated circuits (OEICs), optical-electrical mixed wiring boards, hydride integration devices, multi-chip-module (MCM) devices, plastic optical fibers, etc.
Of the commercially available optical linear high polymer materials, inorganic crystals such as lithium niobate (LiNbO3) are well known and frequently used. Although widely used as a useful material of various optical devices, inorganic crystals have a limitation in mass production because of difficulties in preparation and processing.
Currently, organic high polymer materials that are feasible for flexible design and processing are under consideration to solve the problem with the inorganic materials. Among the organic high polymer materials, polyimides have been conventionally used as protective buffer agents for semiconductors due to their superior thermal and mechanical properties. An approach to making use of polyimides as an optical polymer material while reducing absorption loss involves substitution of fluorine or deuterium for hydrogen in C—H bonds of the polyimides. High polymer materials typically absorb light in the near infrared region ranging from 1000 nm to 1700 nm. Such an absorption loss results from harmonic overtones at high frequencies caused by stretching and deformation vibrations of C—H bonds in the main chain of the polymer. In an attempt to reduce absorption loss, the hydrogen of C—H bonds is substituted with fluorine or deuterium and the absorption wavelength is thereby shifted out of the near infrared region.
Deuterium-substituted polyimides are not suitable as a material for optical communication devices due to absorption occurring at 1550 nm, whereas fluorine-substituted polyimides are now exploited as a promising material for optical communication devices because they enable minimization of absorption loss at 1000 to 1700 nm. In view of providing a difference in refractive index between core and clad layers, the fabrication of an optical wave guide using the fluorine-substituted polyimide usually involves copolymerization of fluorine-containing monomers with fluorine-free monomers at an appropriate mixing ratio in forming core and clad layers.
However, the related art fluorine-substituted polyimides, i.e., the fluorinated polyimides, have a low refractive index due to the fluorines included therein and their use for the core layer results in a narrower selection range of the material for the clad layer, which must have a lower refractive index than the core layer. Use of the fluorinated polyimides for the clad layer also prevents a problem of increasing absorption loss because the number of C—H bonds increases with the reduced fluorine content.
Furthermore, the related art polyimides have a flexible chain structure with less double refraction (birefringence) so that when the etching depth is large during an etching process, a crack occurs in the thin film due to a difference in the coefficient of thermal expansion between a silicon substrate and the polyimides.
Examples of polyimides of the contemporary art are seen in the following U.S. Patents. U.S. Pat. No. 4,385,165, to Ahne et al., entitled POLYIMIDE, POLYIDSOINDOLOQUINAZOLINE DIONE, POLYOXAZINE DIONE AND POLYQUINAZOLINE DIONE PRECURSOR STAGES AND THE MANUFACTURE, describes oligomeric and polymeric radiation-reactive precursor stages of polyimides.
U.S. Pat. No. 4,988,795, to Uekita et al, entitled AMPHIPHILIC POLYIMIDE PRECURSOR AND PROCESS FOR PREPARING THE SAME FROM FATTY-SUBSTITUTED POLYAMIDE-ACIDS, describes amphiphilic polyimide precursors for the preparation of Langmuir-Blodgett films.
U.S. Pat. No. 5,094,517, to Franke, entitled POLYIMIDE WAVEGUIDES AS OPTICAL SENSORS, describes waveguides made by coating polyimide on a substrate.
U.S. Pat. No. 5,171,829, to Uekita et al., entitled COPOLYMERIC AND AMPHIPHILIC POLYIMIDE PRECURSOR, PROCESS FOR PREPARING THE SAME AND THIN FILM, discloses copolymeric amphiphilic polyimide precursors.
U.S. Pat. No. 5,449,741, to Ando et al., entitled POLYIMIDE OPTICAL MATERIAL, describes polyimides with perfluorinated repeating groups.
However, the polyimides disclosed in these patents do not address the problem of achieving appropriate refractive index properties while avoiding cracking when the polyimides are used in thin film applications.